Parsing import allows "as" before "if"

[munificent]

I don't get any errors in:

import 'foo.dart' as foo if (dart.library.html) 'other_foo.dart';

main() {
  print(foo.bar);
}

This runs if I run it with dart temp.dart and analyzes without error using dart analyze. The grammar for imports is:

⟨importSpecification⟩ ::=
import ⟨configurableUri⟩ (deferred? as ⟨typeIdentifier⟩)? ⟨combinator⟩*
‘;’

That means that the as clause, if present, must be after any if configurations.

I ran into this issue because dart format will always output the as clause after the if configurations, which then trips up its sanity check because it made a non-whitespace change by reordering those clauses.

I did find at least two libraries in the wild (dotlottie_loader-0.0.4/lib/src/loaders/network_loader.dart and sentry-8.3.0/lib/src/environment/environment_variables.dart) that has the clauses in the wrong order, so fixing this is both nominally and actually a breaking change (though likely a very minor one given the rarity of conditional imports).

[dart-github-bot]

Summary: The Dart parser incorrectly allows the as clause to appear before the if clause in import statements, leading to unexpected behavior and potential breaking changes when using dart format. This issue arises because the grammar specifies that the as clause must follow any if configurations.

[munificent]

I ran into this issue because dart format will always output the as clause after the if configurations, which then trips up its sanity check because it made a non-whitespace change by reordering those clauses.

I have a PR out for the formatter (dart-lang/dart_style#1555) which will simply preserve whatever order the input code had the clauses in, so the formatter isn't blocked on having this fix. It will gracefully handle imports whether they are spec compliant or not.